The present application claims priority to Japanese Application No. P2000-030529 filed Feb. 8, 2000, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to an electromagnetic wave absorber. More particularly, the invention relates to an electromagnetic wave absorber made of a mixture of a magnetic material and a binding material.
2. Description of the Related Art
As electronic instruments have become smaller and the frequencies have increased there has arisen a serious electromagnetic environmental problem. A noise radiated or leaked from electronic component on a printed board, a communication device or the like has a negative influence on other instruments, or an erroneous operation is caused by an electromagnetic wave from the outside. One countermeasure against this, is to change the wiring pattern of a printed board or to use different components. However, this approach has several disadvantages in that the design must be reconsidered, the costs of the parts are high, and the time required to make a product becomes long. On the other hand, an electromagnetic wave absorber that absorbs unnecessary electromagnetic waves causes the noise itself to be reduced. Therefore, the use of an electromagnetic wave absorber has become the main means for attaining a stable function of an electronic instrument or a communication instrument.
However, in recent years, equipment has been increasingly miniaturized, the packaging density of various semiconductor elements mounted on a substrate has been remarkably increased, and space for the arrangement of the electromagnetic wave absorber is decreased even though the electromagnetic environment becomes worse. In order to solve this, it is necessary to raise the electromagnetic wave absorbing power of the electromagnetic wave absorber.
A conventional electromagnetic wave absorber, is formed by producing particles of a spinel-type ferrite sintered body, a hexagonal ferrite sintered body, or a flake-shaped soft metal magnetic material mixed with a resin. Material parameters that affect the characteristics of an electromagnetic wave absorber are the complex dielectric constant xcex5 and the complex permeability xcexc at a high frequency. In an electromagnetic wave absorber using a magnetic material, xcexcxe2x80x3 (imaginary part of the permeability, term of magnetic loss) of the complex permeability xcexc (=xcexcxe2x80x3xe2x88x92j xcexcxe2x80x3) concerns the electric wave absorption characteristics.
Although a magnetic material capable of coping with a high frequency is generally used for the electromagnetic wave absorber, it is necessary to raise xcexcxe2x80x3 as a physical constant for converting electromagnetic wave energy into heat at the frequency. Normally, a material of about 5 to 10 in the GHz band is used. As the electromagnetic wave absorber used for an electromagnetic wave absorbing sheet for an EMC (Electromagnetic Compatibility) countermeasure or for an electromagnetic interference suppressor sheet, a composite magnetic material in which spinel-type ferrite powder or flat soft magnetic material metal powder is mixed with resin is known in the art.
The shape of the magnetic material powder may be a flake shape, a flat shape, a resin shape or a fiber shape. When the powder is made as a disk shape or an elliptical shape and the surface is made smooth, although anisotropy in an in-plane direction is decreased and anisotropy in a plane vertical direction is increased, so that the permeability is increased. As a result, a high permeability up to a high frequency exceeding to Snoek limit (limit of rotating magnetization) can be obtained. As a method of forming such a disk-shaped magnetic material, a method of forming it from a thin film, a method of forming it from a spherical particle, and a method of smoothing its surface have been previously used.
FIG. 5 is a schematic explanatory view showing a method of forming a disk-shaped magnetic material from a thin film.
As shown in the drawing, a disk-shaped magnetic material is obtained by forming a thin film on a base film 1 through a mask 2 by sputtering, evaporation, CVD or the like. The drawing shows an evaporation method by an Ar beam 4, and a target 3 uses a material such as a Fe base magnetic material.
The molten metal is evaporated from the target 3 of the Fe base magnetic material through the mask 2 in which a pattern of a number of holes (not shown) are formed and is adhered to the base film 1. Subsequently, the mask 2 is removed and disk-shaped fine particles 5 of disk-shaped metal magnetic materials are adhered to the base film 1. The disk-shaped fine particles 5 are then peeled off from the base film 1 to form the disk-shaped metal magnetic materials.
FIG. 6 is a schematic explanatory view showing a method of forming a disk-shaped magnetic material from a spherical powder particle. First, spherical particles 7 are formed by an atomizing method or a chemical deposition method. In the chemical deposition method, metal salt of iron is reduced to deposit iron fine particles. In the atomizing method, molten metal is dropped or is blown by a nozzle into a high speed fluid of gas, water or the like, and fin particles are formed by the fluid during a cooling process. The diameters of the spherical particles 7 may be adjusted from several hundreds nm to several tens xcexcm in accordance with design conditions of electromagnetic wave absorber. Such spherical particles 7 are then crushed by applying the physical force of a stamp mill 4 to form flat disk-shaped fine particles 5.
FIG. 7 is a schematic view showing a method of processing the powder magnetic materials, which are formed in FIGS. 5 and 6, by acid. A flake-shaped magnetic material particle 6 that has a surface which includes irregularities or protrusions is immersed in an acid solution so that the surface becomes smooth resulting in a circular flat plate magnetic material 9 having high permeability.
However, in the case where the soft metal magnetic material is formed from the thin film as in FIG. 5, practical application is difficult in view of costs. In the case where the magnetic material is formed from the spherical powder particle as in FIG. 6, it is difficult, because of microscopic irregularities, protrusions or the like, to form a flat metal soft magnetic material having a skin depth or less in which an electromagnetic wave can penetrate. Furthermore, both of the above methods are not optimum forming methods in view of reproducibility or mass productivity. Also, in the method of processing the flake-shaped powder by acid, the yield of complete circular powder is low, and there is a problem in maintaining a uniformity of shape.
The present invention has been made in view of the above related art, and has an object to provide an electromagnetic wave absorber which is improved in uniformity, reproducibility, and productivity by forming a soft metal magnetic material flat plate, which has a smoothed surface, of a regular shape, such as a disk or ellipse easily, stably and at low cost.
In order to achieve the above object, the present invention provides an electromagnetic wave absorber which is made of a mixture of a magnetic material particle and a binding material and is characterized in that the magnetic material particle comprises a nucleus made of an organic material and a magnetic material film formed on its surface.
According to this structure, by forming the magnetic material particle by the nucleus made of the organic material and the magnetic material film formed on the surface, the nucleus of a regular shape disk shape or elliptical flat plate shape can be formed by a synthetic resin material or the like easily and at low cost, and by coating the surface of this nucleus with the magnetic material film, the surface of the magnetic material particle is smoothed and comes to have the regular shape disk or elliptical shape. By this, the permeability as the electromagnetic wave absorber is increased, and the uniformity, reproducibility, and productivity are raised.
A preferred structural example is characterized in that a thickness of the magnetic material film is a thickness of a skin depth or less.
According to this structure, an electromagnetic wave is certainly permeated into the magnetic material film and is absorbed.
The mixture may also be produced as a sheet or as a paste to obtain a form which facilitates an actual use as the electromagnetic wave absorber.
The present invention also provides a method of manufacturing an electromagnetic wave absorber comprising the steps of: forming an organic material into a predetermined shape; and plating a magnetic material film on the surface of the organic material to form a magnetic material particle, such that the magnetic material particle is a shape approximately the same as the predetermined shape.